The present invention relates to control systems; and more particularly, it relates to control systems wherein a number of individual sensors are used, and it is desired to selectively communicate each sensor with one or more control devices associated with that sensor. Control systems of this type may be used, for example, in large buildings where it may be desired to integrate all of the heating, ventilating, cooling, and even burglar alarm systems into a single master system. Control systems of this type have been in demand, and this demand has increased with the desire to manage energy consumption in large buildings. The sensor devices may be any of a large number of sensors, such as a thermostat calling for heat, a humidistat, a sun or light sensor, etc. Further, the control devices have an equally broad range of functions, including any device, alarm, or communication system that can be turned on and off by a switching signal.
Existing systems which provide such remote control take many forms. One of the techniques most frequently used commercially is to employ direct wiring from each sensor to all of its associated control devices. Although a direct wire system may have economic advantages for smaller installations, there is a break even point where for larger installations, the extensive cable and conduit which results becomes more expensive, and the installation costs increase disproportionately. Further, when such a direct wire system is installed in a large building it is expensive to modify it, and it is difficult to troubleshoot it.
Multiplexing systems have been suggested for overcoming the above disadvantages of a direct wire system. Some multiplexing systems are interconnected to a common conductor or utilize the power distribution network as the conductor between devices, control units and sensors. Phase encoding, frequency modulation, amplitude modulation or other encoding techniques or combinations of techniques have been utilized in multiplexing systems to selectively communicate one unit with another unit or a group of other units.
A primary consideration in the design of multiplexing systems of this type is reduction of the possibility of error; and as a result, extensive filtering, redundancy, encoding and other electronic techniques have been incorporated into these systems to minimize the possibility of error. The resulting multiplexing systems are thus expensive, relatively slow to react, inflexible and physically large. Thus, the advantage of these multiplexing systems over direct wire systems has been reduced.
A time division multiplexing communication system is disclosed in Schwartz U.S. Pat. Nos. 3,916,108 and 4,010,326. This system is intended to communicate computer terminals along a common information bus. A plurality of synchronization lines is fed to each terminal, and a unique combination of phase relationships among the synchronization signals is used for addressing each terminal. Another time division multiplexing system is disclosed in Maniere et al U.S. Pat. No. 3,601,534 wherein two parallel and juxtaposed transmission lines are used, one line being used to transmit synchronization signals from a central station and the second line being used to transmit information signals. Counters are used at receivers for counting synchronization pulses and using the count to identify received calls corresponding to an allotted time slot of the overall operating cycle or frame. This system is, however, characterized in that in each sending station, the information signals transmitted along one transmission line are derived from the synchronization signals transmitted along the second so that information is transmitted only in one direction around a loop because the synchronization pulses must propagate with the information signals and be compared at each sending and receiving station. Further, such system requires true transmission lines with distributed impedances to achieve the proper comparison of waveshapes of synchronization and information signals.
The present invention includes a transmitter for each sensor, a receiver for each control device and a Master Synchronizer. A single signal line is connected to all transmitters and receivers; and a single synchronization line couples the Master Synchronizer to all transmitters and receivers. The Master Synchronizer determines the overall time frame or operating cycle of the system, and the signal it transmits includes a periodic signal portion and a reset signal portion. Each cycle of the periodic signal defines a time slot in the time frame, and the reset signal portion is used to reset the Master Synchronizer and all transmitters and receivers at the same time, thereby achieving overall synchronization. Each transmitter and one or more associated receivers are allocated a predetermined time slot of the time frame, during which time slot the transmitters communicate with their associated receivers.
A decrementing counter is provided for each transmitter and each receiver. At the beginning of each frame, these counters are set to a predetermined number representative of the time slot allocated to those units. When the counter is decremented to zero, a time slot is defined for communicating a transmitter with its associated receivers. Thus, a transmitter is permitted to send a signal along the signal line to all receivers associated with it, and this signal causes a response only in those receivers whose counters have been decremented to zero during the same time slot. In the preferred implementation, this communication occurs once every time frame.
The Master Synchronizer also includes a decrementing counter, and when it reaches zero the transmission of synchronization ("sync" for short) signals to all transmitters and receivers is inhibited for a predetermined time. Thus, the absence of sync pulses is used as a reset signal to reset the Master Synchronizer and all transmitters and receivers once each time frame. An energy-containing signal, rather than the absence of a signal, is required to actuate a control device. This reduces error in the event of line interruption or power failure.
Other features and advantages of the present invention will be apparent to persons skilled in the art from the following detailed description of one embodiment of the invention, accompanied by the attached drawing wherein identical reference numerals will refer to like parts in the various views .